Method and apparatus for welding with mechanical arc control

ABSTRACT

A method and apparatus for feeding wire from a source of wire to a weld includes one or more motors disposed adjacent the wire to drive the wire to the weld. A buffer is disposed between the source and an arc end of the torch. Another motor may be disposed near the wire source. A wire tension controller may be provided. The one or more motors advance and retract the wire, and wire is stored, or the path length altered, when the wire is retracted.

This is a continuation of, and claims the benefit of the filing date of,U.S. patent application Ser. No. 10/200,886, filed on Jul. 23, 2002,which issued as U.S. Pat. No. 6,963,048 on Nov. 8, 2005.

FIELD OF THE INVENTION

The present invention relates generally to the art of welding. Morespecifically, it relates to welding using a short circuit process.

BACKGROUND OF THE INVENTION

There are many different arc welding processes used for numerous weldingapplications. While different processes share some characteristics, suchas using an electric arc and/or current flow to provide the heat for theweld, different processes have characteristics that render themdesirable for particular applications.

MIG welding is a widely used process that gives high heat input into thewire electrode and the workpiece, and thus can give high depositionrates. However, the process can be unstable and control of the arclength can be difficult. The MIG process is often performed as a shortcircuit welding.

Another known welding process is called controlled short circuitwelding, or short circuit welding. Short circuit welding is oftenperformed as a MIG process. Generally, short circuit welding includes ashort circuit state, wherein the welding wire is touching the weld poolthus creating a short circuit, and an arc state, wherein an arc isformed between the welding wire and the weld pool. During the arc statethe wire melts, and during the short circuit state the molten metal istransferred from the end of the wire to the weld puddle.

Disadvantages of short circuit welding relate to the transitions betweenstates, and instability of the process. Transition from the shortcircuit state to the arc state was typically caused by providingsufficient current to “pinch” off a droplet. The pinching off at highcurrent can result in a violent disintegration of the molten metalbridge producing excessive weld spatter. Instability also results fromthe weld pool being pushed away.

Many attempts in the prior art were made to create a stable shortcircuit welding power supply, such as those shown in U.S. Pat. Nos.4,717,807, 4,835,360, 4,866,247, 4,897,523, 4,954,691, 4,972,064,5,001,326, 5,003,154, 5,148,001, 5,742,029, 5,961,863, 6,051,810 and6,160,241. These patents generally disclose complicated control schemesthat fail to control the process to provide a stable and effective weld.They include control schemes that try to control the deposition ofmaterial and/or predict or cause a transition to the subsequent statebased on the total energy put into the weld, the length of the stickout, total watts, time of the preceding state, etc.

These schemes share a common failure: they attempt to control both theenergy of the weld and the transition between states using outputcurrent or power. This necessarily entails a sacrificing of one controlgoal (either energy to the weld or state transition) for the sake of theother. The net result is that the control schemes do not perform well ateither controlling the energy into the weld or controlling thetransition.

Another short circuit welding control system is disclosed in U.S. Pat.No. 6,326,591. This system adequately controls the energy into the weld,but it does not provide independent control of the transitions betweenstates.

The present inventors have published descriptions of a controlled shortcircuit welding process where mechanical movement of the wire (advancingand retracting) is used to control the transition between weldingstates. The short circuit state is entered by advancing the wire untilthe wire touches the weld pool. The arc state is entered by retractingthe wire until the wire does not touch the weld pool, and an arc forms.This system allows a typical output control to be used to control theenergy delivered to the weld. By separating control of the transitionsfrom control of energy, the system allows for better control of each.

A controlled short circuit welding system requires the capability ofadvancing and retracting the wire. The inventors have disclosed in theliterature the use of a stepper motor to control the wire movement. Astepper motor adequately provides for short term advancing andretracting of the wire.

However, a stepper motor does not necessarily provide adequate feedingof the wire over the long term. Accordingly, a system that provides foradvancing and retracting of the wire, and long term feeding of the wire,is desirable.

One problem with controlled short circuit welding arises when the wireis retracted. The wire from the source is feeding toward the weld, andhas momentum in that direction. The retracting motor moves the wire inthe opposite direction. With nothing to compensate for the opposingforces, the wire might not feed in a smooth and efficient manner.Accordingly, a controlled short circuit welder that compensates for thereversal of the wire is desirable.

Another problem with controlled short circuit welding is that the priorart has not fully taken advantage of the process control made possibleby the mechanical control of the state transitions. Thus, a controlledshort circuit welder that provides for electrical control of the arc forthe purpose of controlling heat into the weld, and not for causingtransitions from one state to another, is desirable.

The prior art has not adequately addressed the needs of short circuitwelding at lower currents with thicker wires. The difficult to implementcontrol schemes, in particular, make it difficult to weld with thickerwire, such as 2.4 mm diameter wire, e.g., at low currents, such as lessthan 100 amps. Accordingly, a controlled short circuit welding processthat may be used at low currents relative to the wire diameter isdesirable.

SUMMARY OF THE PRESENT INVENTION

According to a first aspect of the invention a system for feeding wirefrom a source of wire to a weld includes one or more motors disposedadjacent the wire to drive the wire to the weld. A buffer is disposedbetween the source and an arc end of the torch.

According to a second aspect of the invention a system for feeding wirefrom a source of wire to a weld includes at least one motor disposedadjacent the wire and disposed to drive the wire to a torch. A wiretension controller is provided.

According to a third aspect of the invention a method of feeding wirefrom a source of wire to a torch includes driving the wire toward andaway from the torch and, when the wire is driven away from the torch,storing a length of wire between the source and an arc end of the torch.

According to a fourth aspect of the invention a method of feeding wireto a welding system includes driving the wire toward and away from anarc end of a torch and controlling the tension in the wire.

According to a fifth aspect of the invention a method of feeding wirefrom a wire source to a torch in a welding system includes driving thewire toward and away from an arc end of the torch and, when the wire isdriven away from the torch, changing the length of the wire path fromthe source to an arc end of the torch.

According to a sixth aspect of the invention a system for feeding wirefrom a source of wire to a weld includes at least one motor disposedadjacent the wire and disposed to drive the wire to a torch and a sensordisposed to sense the tension in the wire.

The buffer includes a wire liner, and the wire is in the liner at leastpart way from the source to the torch. The liner may be compressible,such as a coil spring in various embodiments.

The liner is in a tube, such that the path of the liner within the tubecan be increased or decreased, relative to the tube length, for examplethe tube has an inner diameter substantially greater than the outerdiameter of the liner in other embodiments.

The buffer includes a shaft with a hollow path therethrough, and thewire extends though the hollow path. The tube is mounted to the shaft,so that the tube moves with the shaft relative to the wire, or it doesnot move, in various embodiments.

Sensors are provided, that sense the amount of wire in the buffer, orthe tension on the wire, for controlling the speed of the wire fed, orthe length of the wire stored, or the tension therein, in responsethereto in various alternatives.

The one or more motor is one of a stepper motor, a servo motors, a zerobacklash motor, or a gearless motor in other alternatives.

A wire feed motor is disposed closer to the source than to the torch,and the stepper motor, a servo motors, a zero backlash motor, a gearlessmotor, or a linear actuator closer to the torch than the source in someembodiments.

The at least one motor may be a pair of motors disposed on oppositesides of the wire.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a welding system, in accordance with the presentinvention;

FIG. 2 is a torch with a buffer and reversible motors in accordance withthe present invention;

FIG. 3 is a cross-sectional view of the torch of FIG. 2;

FIG. 4 is a detailed cross-sectional view of a buffer in accordance withthe present invention;

FIG. 5 is a cross-sectional view of a weld cable used as part of abuffer in accordance with the present invention; and

FIG. 6 is a wave form of a process cycle in accordance with thepreferred embodiment.

Before explaining at least one embodiment of the invention in detail itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting. Like referencenumerals are used to indicate like components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention will be illustrated with reference to aparticular welding system using particular components, it should beunderstood at the outset that the invention may also be implemented withother systems, components, and modules, and be used in otherenvironments.

Generally, the present invention is a method and apparatus forcontrolled short circuit welding that includes mechanical control oftransitions between the arc and short circuit states. In one embodimentthe process includes a pulse mode. Control of energy to the weld iseffected using the output current or voltage magnitude, wave shape,time, etc. Thus, the transitions are caused to occur, and current can becoordinated with, the transitions to reduce spatter, instability, orother undesirable features, by, for example, changing the current as thetransition occurs, or in anticipation of the transition.

Mechanical control of the states is performed by advancing andretracting the wire at the arc. An advance followed by a retractiondefines one process cycle. (Process cycle, as used herein, includes onecycle of the states of the process such as an arc state followed by ashort circuit state, or an arc state, followed by a short circuit state,followed by a pulse state, etc.) The advancing and retracting are, inthe preferred embodiment, accomplished using a pair of motors disposedon either side of the wire, opposite one another and near (or mountedon) the torch. The motors are, in various embodiments stepper motors,servo motors, planetary drive motors, zero backlash motors, gearlessmotors, or replaced with a linear actuator. The pair is disposed oneafter the other in one embodiment.

Stepper motors are used in the preferred embodiment, and the number, andangle or size of the step is controlled to control the length of wireadvanced or retracted.

The preferred embodiment includes a wire feed motor mounted near thesource of wire, such as a reel of wire, that drives the wire to thetorch (although other embodiments omit this motor). As the reversiblemotors retract the wire (and the wire feed motor continues to feed thewire) a buffer is provided to account for the increase in wire betweenthe wire feed motor and the reversible motors. Similarly, when thereversible motors advance the wire, wire is withdrawn from the buffer.The reversible motors move the end of the wire in addition to themovement from the wire feed motor, or they superimpose motion ontomotion imposed by the wire feed motor. The speed of the wire feed motoris slaved to the average speed of the reversible motors, so that, onaverage, they both drive the same length of wire, in the preferredembodiment.

The buffer may be anything that stores and returns the extra wire, orprovides an increased wire path length between the source and the torch.The buffer of the preferred embodiment includes a wire liner about thewire for at least a portion of the distance from the source to thetorch. The liner is disposed in a tube that is wider, and the liner canbend and flex within the tube, thus increasing the length of wire in agiven length of tube. The tube is mounted to a hollow shaft, and thewire passes through the shaft. The shaft is fixed in one position. Thus,as the wire is retracted, the wire moves relative to the tube and shaft(or the tube and shaft may be said to move relative to the wire). Theshaft could be mounted to slide along the axis of the wire, and thusmove relative to the tip of the torch, thereby increasing the length ofthe wire path between the tip (arc end) of the torch and the wire sourceend of the torch.

Alternatively, the liner may be mounted to the shaft, and the wire movesrelative to the liner. The liner is compressible, such as a coil spring,so that as the wire retracts, the spring compresses, in the preferredembodiment. Sensors may be provided that sense the amount of wire in thebuffer, or the tension of the wire, and the process controlled (averagewire feed speed e.g.) may be controlled in response thereto.

A controller is provided that causes the motors to reverse at least onceper process cycle in the preferred embodiment, and controls the currentoutput based on mean arc current (average current during the arc stateonly, or a function thereof), power, energy, voltage, or other weldingoutput parameters. Feedback may include one or more of short detection,buffer feedback, tension feedback, pool oscillation, in addition totraditional welding parameters. Alternatives include reversing lessfrequently than once per cycle. One alternative provides for repeatedreversals during the weld (i.e., not merely at the conclusion of theweld), but not once per cycle.

For example, the braking at the end of the arc cycle can feed forcesbetween wire and droplet, which may disrupt the liquid bridge withoutretracting action. This is particularly present with lower wirediameters and higher short circuiting frequencies. The droplet has thespeed of the wire before braking. This kinetic energy can be enough fordisrupting the liquid path. In this case, no retracting is needed.

The control may include controlling heat, penetration and/or beadformation by controlling the advancement of the wire into the weld pool.The relative time in arc state and short state (arc balance) may be setby the user (as may be the time in the pulse state if it is used).Control of parameters such as polarity (balance), gas mixtures etc. maybe done in coordination with the relative arc/short times (or otherparameters).

Referring now to FIG. 1, a welding system 100 includes, in accordancewith the preferred embodiment, a power supply 102, a wire feeder. 104, acontroller 106 and a torch 108, and a supply line 112 which feedswelding current, gas, water, control, and current for motors to torch108, that cooperate to provide welding current on weld cables 105 and107 to a workpiece 110. Power supply 102, wire feeder 104 and controller106 may be commercially available welding system components, such as aMiller Invision 456® power supply, and a modified Miller XR® wirefeeder. Power supply, as used herein, includes any device capable ofsupplying welding, plasma cutting, and/or induction heating powerincluding resonant power supplies, quasi-resonant power supplies, etc.,as well as control circuitry and other ancillary circuitry associatedtherewith. Power source, or source of power, as used herein, includesthe power circuitry such as rectifiers, switches, transformers, SCRs,etc. that process and provide the output power. Wire feeder, as usedherein, includes the motor or mechanism that drives the wire, themounting for the wire, and controls related thereto, and associatedhardware and software. It can include a motor near the source of wirethat pushes the wire to the weld, and/or motor(s) near the torch thatpulls the wire into the line and to the contact tip, or pulls the wireback from the contact tip. Wire path as used herein, includes the paththe wire takes from the wire source to the torch or power supply, andmay include through a liner, a buffer, etc.

Controller 106 is part of wire feeder 104 and power supply 102 in thisembodiment. Controller 106 also includes control modules adapted for thepresent invention, such as a reversible wire feeder control module tocontrol the reversible motors, a mean arc current module, and thecontrol module for the mechanical control of the arc states. Controller,as used herein, includes digital and analog circuitry, discrete orintegrated circuitry, microprocessors, DSPs, etc., and software,hardware and firmware, located on one or more boards, used to control adevice such as a power supply and/or wire feeder. Control module, asused herein, may be digital or analog, and includes hardware orsoftware, that performs a specified control function. For example, amean arc current control module controls the output to provide a desiredmean arc current.

FIG. 2 shows torch 108 in more detail. Torch 108 includes, in additionto the features of prior art torches, a pair of motor housings 203 and205 have motors disposed within to drive the wire to or from the weld,and a buffer 201 to take up wire 209 when it is retracted, and providewire 209 when it is advanced. Buffer, as used herein, includescomponents used to take up the wire when the wire direction is reversedand provide wire when the wire is advanced. The end of the wire at thearc is shown as 207. The motor housings and buffer are adjacent to thetorch in the preferred embodiment, and near the torch in otherembodiments. Adjacent the torch, as used herein, includes abutting,touching or part of the torch, directly or through a housing. Near thetorch, as used herein, includes much closer to the torch than the sourceof wire, such as more than 75% of the way from the source to the torch.One embodiment provides that a hand held torch includes a small spool ofwire mounted on the torch.

FIG. 3 is a cross-sectional view of the torch of FIG. 2, taken alonglines A—A. A pair of motors 301 and 302 are preferably stepper motors(although they may be other motors) and drive the wire and are disposedadjacent to the wire, and directly opposite one another, on oppositesides of the wire, thereby substantially equalizing forces on the wire.In alternative embodiments they are disposed one following the other, oron the same side of the wire. Directly opposite one another, as usedherein, includes at substantially the same position along a wire path.Disposed adjacent the wire, as used herein, includes being close enoughto the wire to push or pull the wire. Drive the wire, as used herein,includes one or both of moving the wire toward the torch and moving thewire away from the torch.

Buffer 201 may also be seen on FIG. 3, and is shown in more detail onFIG. 4, and includes a shaft 401 mounted on a support 403. Shaft 401 hasa hollow axis, through which wire 209 passes. Weld cable 105 (FIGS. 1and 5) is comprised of an outer tube 501 and a liner 503, with wire 209disposed therein. The outer diameter of line 503 is substantiallysmaller than the inner diameter of tube 501, to allow for wire length tobe taken up or stored by liner 503 flexing within tube 501. Liner 503 ispreferably a coil spring that allows for compression and expansion tofurther buffer the wire. Storing a length of wire, as used herein,includes taking up wire when the wire direction is reversed.Substantially more than an outer diameter of the liner, as used hereinincludes enough room to move and flex. Wire liner, as used herein,includes a tube in which the wire can easily move. Tube 501 is mountedto shaft 401 so that wire 209 moves with respect to shaft 401.

A sensor can be included that senses the amount of wire taken up bybuffer 201. Examples of such sensors include a wheel with an encoderthat is turned as the wire moves past it, or a linear transformer, withthe liner being comprised of a ferrite or magnetic material. Thecontroller includes a buffer feedback input that receives the feedback,and provides a wire feed motor output that is responsive to the bufferfeedback. Tension in the wire can also be sensed and used to control theprocess.

Control of the process from an electrical standpoint is easier sinceprocess control is performed using mechanical control of the wireposition. Therefore, the welding current becomes an independent processparameter, totally opposite to the conventional MIG process.

One desirable control scheme uses mean arc current (average currentduring the arc state, or a function thereof) as the control variable.This allows better control of the melting and heat to the weld, andreduces spatter and instability, compared to prior art control schemes.It is possible to use mean arc current to control the heat, since arccurrent is not used to cause the transition from arc to short (or theopposite). The control of the states can be coordinated with the currentcontrol. For example, if a state transition is to occur at a time T1,the current transition can occur shortly before that, so as to avoiddisrupting the weld pool. Another control feature is to allow the userto set relative arc and short time, or balance between EP and EN.

One desirable arc waveform is shown in FIG. 6, and includes an arccurrent waveform with three segments—an initial high current segment, anintermediate current segment, and a low current segment. The low currentsegment is entered into prior to the short forming, thereby enhancing asmooth transition to the short circuit state.

Because the welding current becomes an independent process parameter,the current can be set to the value, which directs the process into thewanted situation by physical determined behavior. For a low spattermaterial transfer, the forces onto the liquid have to be low, when thecross section of the electrical conductor is low. Therefore, thecurrents have to be low during those phases. During the middle part ofthe short circuit state, where larger cross section of the electricalconductor is present, high forces can be used to move liquids. Also,high currents during the middle part of the short circuit state arepossible. During the arc phase, the current can be used for movement ofthe liquid and determining the melting rate.

The present invention may be used with known control schemes, butimplement them in a more desirable fashion by eliminating the need forcurrent levels to cause transitions. For example, schemes using eitherarc length or stick-out as a control variable can be implemented easilybecause the stepper motors allow stick-out to be measured precisely.Because the transitions are caused mechanically, the arc length may beredetermined each process cycle.

The present invention may be implemented with a variety of processes,including but not limited to electrode positive, electrode negative,alternating polarity, ac mig, mig brazing, hard facing, and welding withthick wire at low currents. For example, welding on a 2.4 mm wire may beperformed at 100 amps, or even 35 or fewer amps with the presentinvention. Prior art systems required more current on thick wire tocause the short to clear and to enter the arc state. The presentinvention doesn't rely on current to clear the short, so thick wire andlow current may be used.

The control preferably ties the speed of the wire feed motor to theaverage speed of the stepper motors, so that the wire feed speed followsthe process speed. Averaging speed over 20–30 process cycles (about 500msec.) provides for effective control.

Pool oscillation frequency can be found by monitoring the distance thewire travels until a short is created, or an arc is created. One controlscheme provides that the state transitions are timed to coincide withthe natural frequency of pool oscillation. The controller includes afrequency module and a pool oscillation feedback circuit that effectthis control scheme. A short detection feedback circuit may be used aspart of the control loop.

Numerous modifications may be made to the present invention which stillfall within the intended scope hereof. Thus, it should be apparent thatthere has been provided in accordance with the present invention amethod and apparatus for controlled short circuit welding that fullysatisfies the objectives and advantages set forth above. Although theinvention has been described in conjunction with specific embodimentsthereof, it is evident that many alternatives, modifications andvariations will be apparent to those skilled in the art. Accordingly, itis intended to embrace all such alternatives, modifications andvariations that fall within the spirit and broad scope of the claims.

1. A method of feeding wire from a source of wire to a torch,comprising: advancing the wire toward the torch causing a short circuitcondition to be entered into and retarding the advancing of the wiretoward the torch causing an arc condition to be entered into; and whenthe advancing is being retarded, storing a length of wire between thesource and an arc end of the torch, by moving the wire relative to aliner disposed about the wire and within an outer structure, such thatthe path of the liner within the structure is increased or decreased,relative to the structure length.
 2. The method of claim 1, wherein thestructure is a tube having a shaft with a hollow path therethrough, andmoving includes moving the wire in the hollow path.
 3. The method ofclaim 1, further comprising sensing the length of wire stored.
 4. Themethod of claim 3, further comprising controlling the speed of the wirefed in response to the length of wire stored.
 5. The method of claim 3,further comprising controlling the length of wire stored in response tothe length of wire stored.
 6. A method of feeding wire from a wiresource to a torch in a welding system comprising: advancing the wiretoward an arc end of the torch causing a short circuit condition to beentered into and retarding the advancing causing an arc condition to beentered into; and when the wire advancing is being retarded, changingthe length of the wire path from the source to an arc end of the torch,by moving the liner within an outer structure, such that the path of theliner within the outer structure is increased or decreased, relative tothe length of the outer structure.
 7. The method of claim 6, furthercomprising sensing the length of wire stored.
 8. The method of claim 7,further comprising controlling the speed of the wire fed in response tothe length of wire stored.
 9. The method of claim 7, further comprisingcontrolling the length of wire stored in response to the amount ofchange in the length.
 10. The method of claim 6, further comprisingsensing the tension of the wire somewhere between the source and the arcend of the torch.
 11. The method of claim 6, further including feedingwith at least one motor disposed adjacent the wire closer to the sourcethan to the torch, and driving includes driving with a stepper motor, aservo motors, a zero backlash motor, a gearless motor, or a linearactuator closer to the torch than the source.
 12. A system for feedingwire from a source of wire to a weld, comprising: means for providingwire; means for driving the wire toward an arc end of the torch causinga short circuit condition to be entered into and for retarding thedriving causing an arc condition to be entered into; and means forstoring a length of wire between the source and the arc end of the torchwhen the wire is driven away from the arc end of the torch, includingmeans for moving the wire relative to a liner disposed about the wireand disposed within an outer structure, such that the path of the linerwithin the outer structure is increased or decreased, relative to theouter structure length.
 13. The system of claim 12, wherein the meansfor storing includes means for moving, relative to wire, the liner and ashaft with a hollow path therethrough, wherein the wire follows thehollow path.
 14. The system of claim 12, wherein the means for storingincludes means for compressing the liner.
 15. The system of claim 12,further comprising means for sensing the length of wire stored,connected to the means for storing.
 16. The system of claim 12, furthercomprising means for controlling the speed of the wire fed in responseto the length of wire stored.
 17. The system of claim 12, furthercomprising means for controlling the length of wire stored in responseto the length of wire stored.
 18. The system of claim 12, furthercomprising means for sensing the tension of the wire somewhere betweenthe source and the arc end of the torch.
 19. The wire feeder of claim12, wherein the means for driving includes at least one of a steppermotor, a servo motors, a zero backlash motor, or a gearless motor. 20.The wire feeder of claim 12, wherein the means for driving includes amotor disposed adjacent the wire includes a wire feed motor closer tothe source than to the torch, and at least one of a stepper motor, aservo motors, a zero backlash motor, a gearless motor, or a linearactuator closer to the torch than the source.
 21. A welding-type powersupply, comprising: a source of welding-type power, disposed to providewelding-type power to a torch; a wire feed motor disposed near a sourceof wire and further disposed to engage the wire and advance the wiretoward the torch; a second wire feed motor, disposed near the torch andfurther disposed to advance and retract the wire, thereby causingsuccessive arc and short states to be entered into; and a buffer,disposed between the wire feed motor and the second wire feed motor,including an outer structure, wherein the wire is disposed within aliner, and the liner is at least partially disposed within the outerstructure, and wherein the path length of the wire within the outerstructure increases as the wire is retracted; and a controller, having aplurality of control outputs connected to the welding-type power supply,the wire feed motor, and the second wire feed motor.
 22. A method offeeding wire from a source of wire to a torch, comprising: advancing thewire toward the torch causing a short circuit condition to be enteredinto and retarding the advancing of the wire toward the torch causing anarc condition to be entered into; and when the advancing is beingretarded, storing a length of wire between the source and an arc end ofthe torch, by moving the wire relative to a liner disposed about thewire and within an outer structure, such that the path of the wirewithin the structure is increased or decreased, relative to thestructure length.